1. Field of the Invention
This invention relates to a method and apparatus for wireless communications and, more particularly, to adaptive transmission in ultra-wideband communication systems.
2. Description of the Related Art
Ultra-Wideband (UWB) communication systems generally involve wireless transmission having a 3 dB bandwidth occupying more than 25% of its center frequency, or more than 1.5 GHz. An Ultra-Wideband Impulse Radio (IR) system communicates using baseband pulses of very short duration—typically on the order of a nanosecond—thereby spreading the energy of the radio signal very thinly over a broad bandwidth. Energy spreading of about a few W per MHz occurs in these systems from the low end of the band near DC to the top end of the band located around a few Gigahertz. For example, see R. A. Scholtz, “Multiple Access with Time-Hopping Impulse Modulation,” Proc. Of IEEE Milcom., Vol. 2, pp. 447-450 (October 1993).
UWB IR communications promises to be a viable technique for building relatively simple, low-cost, low-power transceivers that can be used for short range, high speed, multiple-access, multimedia communications over the multi-path indoor wireless channel. Proponents of UWB communications claim that UWB is intended to reutilize spectrum by taking advantage of light usage of frequency bands assigned to other services. This wideband communication technology is designed to coexist with many other narrow-band signals that simultaneously occupy portions of this extremely large transmission bandwidth without causing intolerable interference problems.
It is important in such a wideband technology that the channel capacity be used efficiently and effectively. But it can be shown that, when the system parameters are designed, optimized, and implemented, the system performance as a measure of channel capacity utilization varies as the channel conditions vary. In the fading wireless environment, channel capacity in a UWB IR system is used relatively efficiently under poor channel conditions and relatively poorly when fading abates.
Conventional multiple access employing time hopping sequences for UWB communications involving IR modulation uses time diversity to achieve a desired bit-error-rate consistent with regulatory restrictions on power spectral density. Unfortunately, the use of time diversity reduces the maximum data rate achievable for the system.